Hi Christian,
Not sure about the AV9000, but if i remember correctly, there were 3(or 4) versions of the AV9000,
of which only the first version (26xx) was 2-way (Beolink 5000)
The other versions (43xx, 79xx were 1-way (Beo4)
Beolink 7000 works fine with Beomaster 4500, including timer programming / lights.
No real limitations, it is just that f.e. a Beosystem 6500 / 7000 has more functions
compared to Beosystem 4500, so naturally, when used with a 6500/7000,
more information / functions on Beolink 7000
I use both Beolink 5000 as well as Beolink 7000 with Beomaster 4500.
The Beolink 7000 shows additional information that normally isn't available,
f.e. the frequency whilst tuning the radio (the Beolink 5000 would only show : TUNE UP)